1. Field of the Invention
This invention is related to optical fibers, and more particularly, to optical fibers with an overclad layer having a low Young's modulus and high water repellency characteristics.
2. Discussion of the Related Art
In the last several decades, fiber optic communication has become ubiquitous. Fiber optic manufacturing technology has matured, and millions of miles of fiber have been laid all over the world, connecting a variety of communication devices. An optical fiber operates by transmitting coherent light, such as light from a modulated laser diode, down its glass core. The cladding, which is around the glass core, has a lower refractive index than the core, allowing light from the laser diode to propagate down the core. Around the cladding of a conventional fiber is the overclad.
The cladding has to perform a number of functions. One purpose of having the cladding around the glass core is to protect the fiber core from various environmental hazards, such as water penetration, or from micro-cracking, which degrades the performance of the fiber, and therefore degrades the transmission properties of the entire fiber optic system. The cladding protects the fiber from moisture and from potential humidity penetration through the cladding, since such moisture penetration can generate defects in the fiber, particularly when mechanical stresses are applied to the fiber. These mechanical stresses can include tensile stress, bending, and twisting of the fiber as the fiber is laid. It is therefore desirable to have a cladding with as low a Young's modulus as possible, to enable the fiber to be as flexible as possible. At the same time, it is desirable to have a cladding that is hydrophobic, so that moisture and water do not penetrate the cladding into the fiber core.
The function of the overcladding is mechanical protection of the fiber core from external impacts and hazards, while the primary function of the cladding is optical (i.e., providing a medium of a lower index of refraction than the fiber core).
Silica glass is by far the best fiber optic material today. Silica glass is frequently used as both the fiber core material (often in the form of doped silica), as well as for cladding (as “regular” silica, or undoped silica). Silica glass is also often used as the “overcladding” material. However, silica glass, despite its widespread use, suffers from a number of disadvantages. One major disadvantage is insufficient fracture resistance (often referred to as “static fatigue,” or a short time until delayed fracture appears after an appreciable thermal or mechanical load is applied to the fiber).
Another shortcoming of the silica glass material is moisture sensitivity, since even a very minute amount of moisture on the fiber surface can result in a sudden failure, particularly in the presence of surface micro-cracks. Yet another major shortcoming of the silica glass material is its inability to withstand appreciable deformations due to tension or bending.
In addition to the use of silica glass claddings, other coatings are applied to the fiber core and cladding. These include non-hermetic, or polymer, coatings; and hermetic (carbon or metal) coatings. Polymer coatings have an advantage in that they have fairly low Young's moduli, and are therefore able to withstand large tensile or bending deformations. Polymers, however, have a major disadvantage in that they absorb moisture. Thus, long-term reliability of a polymer-coated fiber is believed to be suspect. Hermetic coatings, (e.g., metallic coatings) on the other hand, provide good protection from moisture, but have a major disadvantage in being subject to corrosion, and fairly high Young's moduli, which results in poor performance when mechanical stress, such as bending or twisting, is applied. Phosphorous nickel, for example, has a Young's modulus that is approximately three times higher than the Young's modulus of silica glass. Due to its very high Young's modulus, it is not uncommon for the coating, rather than for the silica glass in the fiber core, to fail first, when subjected to mechanical loading.
Accordingly, there is a need for a cladding material that overcomes the disadvantages described above, i.e., in a material that has a low Young's modulus, has good mechanical stability, and at the same time is hermetic or hydrophobic, so as to prevent moisture from penetrating into the fiber.